It all depends on how 'official' you want your army to be. I don't think you need any more daemon princes, since the greater daemons can easily stand in for them if need be. The army looks fine, although if you're looking for somewhere to expand, I'd consider getting some or all of the following:

1. One more soulgrinder. I'd try and run 3 soulgrinders, to make it more difficult for your opponent to destroy them all with his anti-vehicle shooting.

2. More bloodcrushers. You could always convert them out of spare parts if you have any. Bloodletters riding something large is fine (mine are riding the dwarven rolling altars from the Skull Pass starter set).

3. More fiends. Again, you can always convert them. I made mine out of chaos spawn and extra bits, which is significantly cheaper than buying the metal versions.